The present invention relates generally to fiber-reinforced composites wherein the matrix consists of a ceramic material, and more particularly to ceramic matrix composites wherein the reinforcing fibers are silicon carbide fibers and the matrix is an alkaline earth aluminosilicate glass or glass-ceramic.
Fiber-reinforced ceramic matrix composites comprising glass and/or glass-ceramic matrices are well known. U.S. Pat. No. 4,626,515 for example, discloses fiber-reinforced composites wherein the reinforcing fibers are composed of silicon carbide and the reinforced matrix is an essentially alkali-free, alkaline earth aluminosilicate glass. U.S. Pat. No. 4,615,987 discloses silicon carbide fiber reinforced glass-ceramic composites wherein the matrix consists of an alkaline earth aluminosilicate glass-ceramic composition. Similar silicon-carbide-reinforced composites wherein the matrix consists of a barium-modified magnesium aluminosilicate glass-ceramic are reported in U.S. Pat. No. 4,589,900, while U.S. Pat. No. 4,755,489 discloses reinforced glass-ceramics containing anorthite and mullite or alumina.
Prospective uses for fiber-reinforced ceramic matrix composites such as described in these and other prior patents and literature include use as a structural element in high temperature environments such as heat engines. Thus the materials to be employed must not only exhibit good strength and toughness at ambient temperatures, but must also retain those desirable physical properties at the elevated temperatures encountered in the operating environment. Temperatures in the range of 700.degree.-1000.degree. C. and highly oxidizing conditions (due to the high-temperature activity of oxygen) are considered representative of the operating conditions to be encountered.
An important problem which has been identified in silicon carbide reinforced ceramic matrix composites in this temperature regime is that of high temperature embrittlement. Hence, instead of exhibiting high toughness and strength after exposure to temperatures in the operation ranges desired, these materials become brittle and subject to sudden catastrophic breakage, rather than more gradual failure as typical of the original material. While the exact mechanism of embrittlement has not been fully explained, oxidative deterioration of the fiber-matrix interface is the probable cause. See, for example, R. L. Stewart et al., "Fracture of SiC Fiber/Glass-Ceramic Composites as a Function of Temperature", in Fracture Mechanics of Ceramics, R. C. Bradt et al. Ed., Volume 7, pages 33-51, Plenum (New York) 1986.
It is known to provide coatings on fiber materials to be incorporated in composite materials in order to modify the behavior of the materials or the fibers therein. U.S. Pat. No. 4,276,804, for example, describes the application to carbon fibers of metal oxide films for the purpose of improving fiber adherence to a metal matrix.
U.S. Pat. No. 4,397,901 describes a composite article and method for making it wherein a woven or nonwoven fiber substrate, typically composed of carbon fibers, is provided with successive coatings of pyrolytic carbon, diffused silicon, and silicon carbide to provide a composite article resistant to corrosive conditions. U.S. Pat. No. 4,405,685 describes a similar coating system for carbon fibers wherein an inner coating consisting of a mixture of carbon and a selected metal carbide, in combination with an outer coating consisting solely of the metal carbide, are described. This dual coating system is intended to provide enhanced fiber protection for fibers to be embedded in ceramic or particularly metal matrix materials.
U.S. Pat. No. 4,481,257 discloses silicon carbide monofilaments coated with boron or boron carbide. These filaments exhibit improved strength and bonding when used with metal or epoxy matrix materials. U.S. Pat. No. 4,485,179 describes the use, in a ceramic matrix composite comprising silicon carbide fibers, of an agent added to the matrix to reduce interaction with the silicon carbide fibers. Tantalum or niobium compounds are useful for this purpose.
U.S. Pat. No. 4,605,588 discloses a process for providing a boron nitride surface coating on ceramic fibers such as aluminoborosilicate fibers, reportedly effective to reduce reaction bonding of the fiber to glass or ceramic matrices, while U.S. Pat. No. 4,642,271 teaches BN coatings on SiC or alumina fibers. U.S. Pat. Nos. 4,376,803 and 4,376,804, supra, describe composite coatings consisting of amorphous carbon and an overlayer of a metal oxide such as SiO.sub.2 to be applied to carbon fibers in order to improve fiber adhesion to molten metal matrix materials such as magnesium and magnesium alloys. U.S. Pat. No. 3,869,335 describes metal coated fibers and metal-glass coated fibers which can be incorporated into glass matrix materials to provide products which exhibit higher ductility than conventional products.
While the foregoing patents and literature indicate a general interest in the development of coatings for fibers to be employed for the reinforcement of composite glass, metal and ceramic materials, the problem of embrittlement of ceramic matrix composites comprising silicon carbide reinforcing fibers remains.
It is a principal object of the present invention to provide a fiber-reinforced ceramic matrix composite comprising silicon carbide fibers which exhibits improved resistance to embrittlement under adverse high temperature conditions.
It is a further object of the invention to provide a method for making silicon carbide-reinforced ceramic matrix composites which provides products of improved strength and/or toughness at high temperatures.
Other objects and advantages of the invention will become apparent from the following description thereof.